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Towards primary and secondary standards for dosimetry in FLASH radiotherapy

23.12.2021

In the last few years, a new radiotherapy modality, called FLASH radiotherapy, has been investigated and is showing advantages over conventional treatment modalities. To test the FLASH modality and quantify its advantage over conventional treatment, new dose measurement techniques and codes of practice have to be studied and elaborated. For this purpose, the EU funded EMPIR project, UHDPulse, was launched in September 2019. The project aims to develop reliable dosimetry methods for FLASH modality and guidance for code of practice measurement protocols for dosimetry.

In the last few years, a new radiotherapy modality, called FLASH radiotherapy, has been investigated and is showing advantages over conventional treatment modalities [1]. The difference is the amount of dose delivered per unit of time, the dose rate, which is around thousand times higher. This new modality has the advantage that the dose can be delivered in less than one second, compared to several minutes in conventional radiotherapy treatment. The modality is in the early stage of development, but it has already shown to reduce adverse side effects to healthy tissue. The downside of that new modality is that conventional dosimetry methods are no longer suitable for this ultra‑high dose rate [2].

In conventional radiotherapy, the particle accelerators used to deliver the radiation treatment are typically calibrated using dosimeters called ion chambers. The measurement of the radiation is done by counting the number of ions created in the chambers due to radiation interaction. In the FLASH radiation beam, there is a significant reduction of the signal measured since the ions recombine before being collected to be counted. Therefore, ion chambers and the current code of practice protocol are no longer suitable. To test the FLASH modality and quantify its advantage over conventional treatment, new dosimeter technique and codes of practice have to be studied and elaborated. For this purpose, the EU‑funded EMPIR project “Metrology for advanced radiotherapy using particle beams with ultra‑high pulse dose rates” (UHDPulse) was launched in September 2019 [3]. The project aims to develop reliable dosimetry methods for the FLASH modality and guidance for code of practice protocols.

The response of different types of detectors has been measured in an ultra‑high dose‑per‑pulse electron beam at PTB. The detectors used are plane‑parallel ion chambers, alanine pellets and a probe‑type graphite calorimeter [4]. The collection efficiency of ion chambers has been determined for a range of 0.5 to 2 Gy per radiation pulse. The dependence of the collection efficiency in ion chambers was not linear with the dose rate and intra‑type variations of 2‑5% were found as illustrated in figure 1. In figure 2, the depth dose measurement in water with calorimeter and ion chamber is shown in comparison to the Monte Carlo calculation. A good agreement between calorimeter, ion chamber (after the recombination correction has been applied) and the Monte Carlo calculation show that the proper correction factor for the ion chamber was used and, in the case of the calorimeter, no correction factor for heat loss is required in the ultra‑high dose rate pulse.

diagram ion collection efficiency six ionization chambers

Figure 1: Ion collection efficiency of different ionization chambers in ultra‑high dose‑per‑pulse electron beams.

Figure 2: Depth dose measurement in water with calorimeter and ion chamber compared to Monte Carlo simulation.

Although the project is still in early stage, calorimetry is showing promising results for absorbed dose measurements both at national metrological institutes and in clinics. Calorimetry gets simpler at the FLASH dose rate as the dose delivery takes place in a few seconds or less. The preliminary results show that advanced thermal insulation of the calorimeter is not required, nor is the use of a heat loss correction factor [5]. For relative measurements, other types of dosimeters, such as plastic scintillators, diodes, or diamond detectors, are also under investigation to determine the best option.

References

[1]       V. Favaudon et al., “Ultrahigh dose‑rate FLASH irradiation increases the differential response between normal and tumor tissue in mice,” Sci. Transl. Med., vol. 6, no. 245, Jul. 2014, doi: 10.1126/scitranslmed.3008973.

[2]       M. McManus et al., “The challenge of ionisation chamber dosimetry in ultra‑short pulsed high dose‑rate Very High Energy Electron beams,” Sci. Rep., vol. 10, no. 1, pp. 1‑11, Dec. 2020, doi: 10.1038/s41598‑020‑65819‑y.

[3]       A. Schüller et al., “The European Joint Research Project UHDpulse – Metrology for advanced radiotherapy using particle beams with ultra‑high pulse dose rates,” Phys. Medica, vol. 80, pp. 134‑150, Dec. 2020, doi: 10.1016/j.ejmp.2020.09.020.

[4]       J. Renaud, A. Sarfehnia, J. Bancheri, and J. Seuntjens, “Aerrow: A probe‑format graphite calorimeter for absolute dosimetry of high‑energy photon beams in the clinical environment,” Med. Phys., vol. 45, no. 1, pp. 414‑428, Jan. 2018, doi: 10.1002/mp.12669.

[5]       J. Renaud, H. Palmans, A. Sarfehnia, and J. Seuntjens, “Absorbed dose calorimetry,” Physics in Medicine and Biology, vol. 65, no. 5. Institute of Physics Publishing, p. 05TR02, Mar. 02, 2020, doi: 10.1088/1361‑6560/ab4f29.

Contact

Opens local program for sending emailA. Bourgouin, Department 6.2, Working Group 6.21